Engineered substrates can be fabricated according to known techniques such as Smart Cut™ or bond and grind/etch back. Generally, a support is assembled with a donor substrate to form a temporary structure, the thickness of which is then reduced to form the final substrate. Thus, the final substrate often comprises a relatively thin top layer, the remaining thickness of thinned donor substrate, on a relatively thick support. An intermediate layer, e.g., an insulating layer, can be inserted between the top layer and the support. For example, a SOI Silicon On Insulator substrate is formed when the donor substrate and support include silicon and the intermediate layer includes silicon dioxide.
Intermediate insulating layers of significant thickness, e.g., between 200 nm to 3 microns, are common in power device applications where they serve to limit parasitic current flow (dielectric breakdown) arising from the high operating voltages. However, engineered substrates with such thick insulating layers, especially when produced by thermal oxidation of silicon, can be expensive.
Also, the costs of starting materials, i.e., the donor substrate and the support, can contribute to the expense of engineered substrates. For example, since devices are often formed in top layers of engineered substrates which usually originate from donor substrates, donor substrates are often selected to be of higher crystalline quality, and thus also of higher cost.
Lower cost starting materials for engineered substrates are known. In one approach, donor wafers from which thin layers have previously been separated according to Smart Cut™ techniques are reclaimed and reused. Smart Cut™ techniques introduce light ions into donor substrates to form planes of weakness (then, optionally, apply stiffeners to the donor substrates) and fracture the donor substrates at the planes of weakness so as to separate thin layers.
However, the crystalline quality of such reclaimed and reused donor substrates degrades, e.g., the number of defects, especially bonding defects, increases with increasing numbers of reclaims and reuses (the refresh rate). Defects that arise during bonding of a donor substrate to a support (bonding defects) appear in the final substrate as areas where the top layer has not been transferred to a support or where the top layer has been transferred but is weakly adherent. Bonding defects can be readily observed and detected by known observation tools, e.g., KLA-TENCOR SP1™ equipment.
In particular, it has been found that final substrates with a acceptable densities of bonding defects can be formed only from donor substrates that have be reclaimed and reused as a donor or support substrate no more than limited number of times, e.g., 5 or 10 times. An acceptable density of bonding defects can be, e.g., less than 1 per cm or less than 0.1 per cm2, as observed by SP1.
In another approach, the starting materials for supports are initially of lower quality. Since devices are not usually formed in supports, supports do not play an active role in the substrate, and lower quality supports should not impact device performance. A lower quality support can be made from less-expensive polycrystalline materials instead of more expensive mono-crystalline materials, or from materials with greater numbers of defects such as COPs crystal originated particles or oxygen precipitates (perhaps as a result of having been grown under conditions that increase throughput at the expense of quality). COPs are generally defined as tetrahedral voids in a silicon crystal having dimensions from about 10 nm nano-meters to a few 100s of nm.
But is has further been found that bonding defects are also often observed in final substrates fabricated from lower quality supports.
Therefore, despite known cost reduction strategies, engineered substrates remain too expensive for certain applications.